Lecture 4: Membrane Permeability and Membrane Potential

Question 1

What is transmembrane potential (Vm)?

Answer 1

voltage difference across cell membrane

Question 2

What is voltage signal?

Answer 2

difference in electrical potential energy between two points in electrical circuit

Question 3

What is resting membrane potential (RMP)?

Answer 3

voltage across cell membrane at rest (negative inside relative to outside)

Question 4

What is neural electrical signal?

Answer 4

transient change in Vm

Question 5

What is receptor potential?

Answer 5

transmembrane potential difference produced by activation of a sensory receptor (non-electrical stimulus) – detection of touch, sound, light, chemical

Question 6

What is synaptic potential?

Answer 6

potential difference across postsynaptic membrane produced by action of neurotransmitters at a neuronal synapse (the incoming signal that neuron receives) – excitatory or inhibitory

Question 7

What is action potential?

Answer 7

rapid change (rise and fall) in membrane potential produced by impulse along membrane of muscle cell or nerve cell

Question 8

What is depolarization?

Answer 8

Vm becomes less negative than RMP

Question 9

What is hyperpolarization?

Answer 9

Vm becomes more negative than RMP

Question 10

What is repolarization?

Answer 10

Vm returns toward RMP

Question 11

What is Ohm’s Law?

Answer 11

V = IR

Question 12

What is current (I)?

Answer 12

flow of charge from one location to another (can be positive current or negative current)

Question 13

What is resistance (R)?

Answer 13

how difficult (energy-intensive) it is for current to flow

Question 14

What is voltage (V)?

Answer 14

force that provides energy for current flow

Question 15

What does Ohm’s Law predict?

Answer 15

mostly predicts what will happen to neuron’s Vm if you inject a particular current (I) into a neuron – BUT there are two exceptions

Question 16

What are the two exceptions to what Ohm’s Law predicts?

Answer 16

exception 1: if sufficient positive current is injected, AP is triggered once a threshold level of membrane depolarization is reached (L6-7)

exception 2: even with subthreshold current injections, voltage doesn’t perfectly follow the timing of the current (L5)

Question 17

What are the biophysical mechanisms that create electrical membrane potential signals?

Answer 17

ability of cell membranes to generate any transmembrane potential requires:

• unequal distributions of ion species across the two sides of a membrane (concentration differences)
• selective permeability of that membrane to different ion species

Question 18

What does equilibrium mean?

Answer 18

forces pushing movement of ion one way or the other are balanced – ions are still moving across membrane, but Vm is no longer changing

Question 19

What are the forces that establish equilibrium?

Answer 19

diffusive/osmotic pressure/force due to concentration differences

electromotive force due to increase in positive or negative charge on one side (and corresponding excess of negative or positive charge on original side)

Question 20

Is diffusive/osmotic pressure/force or

electromotive force stronger?

Answer 20

EMF is very strong compared to diffusion, therefore net number of ions that must change sides to get to EK is tiny

Question 21

Is equilibrium for a particular ion species always the same?

Answer 21

no – equilibrium for a particular ion species is always specific to the particular cell/system being studied

Question 22

What are the factors that affect electromotive force (EMF)?

Answer 22

• transmembrane potential (Vm)
• ion valence (z)
• Faraday’s constant (F)

Question 23

What are the factors that affect diffusive force?

Answer 23

• concentration gradient (out/in)
• temperature (T)
• gas (diffusion) constant (R)

Question 24

Permeability is necessary to get a transmembrane potential. Why isn’t it part of the Nernst equation?

Answer 24

equilibrium potential represents the final stage – it doesn’t tell you anything about how quickly it will get there, only what the end result would be

it does not matter whether a membrane is very permeable to an ion – it will eventually get to its equilibrium potential

Question 25

When does Vm depend on the relative permeability of the ions?

Answer 25

when more than one ion is permeable

Question 26

In a system where multiple ions are permeable, how will each permeable ion change the membrane potential (Vm)?

Answer 26

each permeable ion will move the membrane potential (Vm) in the direction of its Eion proportionally to its (relative) permeability

Question 27

In the GHK equation, what factors affect all ions equally?

Answer 27

• temperature (T)
• Faraday’s constant (F)
• gas constant (R)

Question 28

In the GHK equation, what factors affect an individual ion’s Eion?

Answer 28

• log([ion]o[ion]i)

- valence/charge (z)

Question 29

In the GHK equation, what factors affect an individual ion’s effect strength?

Answer 29

their relative permeability (Pion)

Question 30

In the GHK equation, where did the z go?

Answer 30

all ions are monovalent

Question 31

In the GHK equation, why are the inside and outside inverted for Cl- ?

Answer 31

to remove the -1 valence from the equation

Question 32

What was the practical use of the GHK equation by neuroscientists?

Answer 32

to try to solve for P, not Vm

Question 33

One Ion

What happens when there is selective permeability, and equal distribution?

Answer 33

charges on each side stay balanced

Question 34

One Ion

What happens when there is selective permeability, and unequal distribution?

Answer 34

build up of charge on one side of membrane (Vm reaches equilibrium potential for that ion)

Question 35

Two Ions

What happens when the permeabilities of two ions are equal (and concentration gradients are equal and opposite)?

Answer 35

no potential develops (Vm = 0 mV)

Question 36

Two Ions

What happens when the permeabilities of two ions are unequal (and concentration gradients are equal and opposite)?

Answer 36

build up of charge on one side of membrane (Vm is closer to Eion of the more permeable ion)

Question 37

What happens if relative permeabilities of two ions vary over time?

Answer 37

Vm will vary predictably over time in proportion to the relative permeabilities at any given time point

hypothesis 1: RMP reflects high relative permeability of whatever ion has closest Eion to RMP (ie. K+)

hypothesis 2: depolarization (ie. during AP) reflects a change toward higher permeability to Na

Question 38

Although equal and opposite ion concentrations produce Vm of 0 mV, are the net ion fluxes balanced?

Answer 38

net ion fluxes are not actually balanced – different ion types are moving in each direction

Question 39

What happens to concentration gradients over time?

Answer 39

concentration gradients run down/equalize

Question 40

What is required in the long-term to maintain concentration gradients?

Answer 40

active transporter proteins

whenever there are ions with opposing equilibrium potentials in a cell, ion homeostasis requires mechanisms that can maintain concentration gradients in the face of slow run-down from ions travelling across the membrane

Question 41

What are active transporter proteins?

Answer 41

pumps in cell membrane that maintain ion gradients over time

Question 42

What does the Na/K-ATPase pump do?

Answer 42

maintains K+ and Na+ gradients (on a time scale of minutes to hours)

for every molecule of ATP hydrolyzed (one cycle), pump moves 3 Na+ out of cell, and brings 2 K+ in

Question 43

Is the Na+/K+ ATPase pump an important determinant of RMP? Why?

Answer 43

NO – it is neither a determinant of any information carrying electrical signal in nervous system, because it does not move enough ions fast enough to matter (ie. within ms)